Doping of inorganic minerals to hydrophobic membrane surface

ABSTRACT

Disclosed is a membrane surface modification method. The method is applicable to a variety of hydrophobic membranes by doping selected inorganic particles. One act of the method involves the in-situ embedment of the inorganic particles onto the membrane surface by dispersing the particles in a non-solvent bath for polymer precipitation. Further membrane surface modification can be achieved by hydrothermally growing new inorganic phase on the embedded particles. The embedment of particles is for the subsequent phase growth.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Division of application Ser. No. 13/217,837, filedon Aug. 25, 2011, which claims priority to Provisional application Ser.No. 61/379,505, filed on Sep. 2, 2010, both of which are incorporatedherein by reference.

TECHNICAL FIELD

Disclosed are surface modification techniques involving doping ofinorganic minerals into polymeric membranes and related uses for waterand wastewater treatment.

BACKGROUND

Filtration techniques utilizing filtering membranes having apermselectivity have made remarkable progress. Filtering membranes arecurrently utilized in practice in numerous applications including, forexample, production of ultrapure water, preparation of medicines,sterilization and finalization of brewages and purification of drinkingwater. The use of filtering membranes is particularly valuable inmeeting the requirement to refine water (a high degree treatment).Furthermore, the quality of surface water and groundwater is gettingprogressively worse as a result of pollution by wastewater and, it isincreasingly recognized as an important way to guarantee the safe use ofwater.

Membrane filtration has a number of applications in water and wastewatertreatment. Most membranes are fabricated from organic polymers which arehydrophobic in nature. Hydrophobic membranes have high tendency to befouled by organic foulants deposition and/or biofilm formation.

Increasing the surface hydrophilicity is often attempted. This istypically realized by the coating of hydrophilic polymer layers,chemical or plasma treatment, graft polymerization, blending ofhydrophilic polymer or amphiphilic copolymer or doping of inorganicmaterials. Doping of inorganic materials is beneficial due to thecompletely hydrophilic characteristic of inorganic materials, as well asthe certain functionality to achieve membrane property modificationand/or fouling reduction. However, the inorganic materials on/in themembranes modified by current available doping approaches are either notstable on the membrane surface or do not appear on the membrane surface.

SUMMARY

The following presents a simplified summary of the invention in order toprovide a basic understanding of some aspects of the invention. Thissummary is not an extensive overview of the invention. It is intended toneither identify key or critical elements of the invention nor delineatethe scope of the invention. Rather, the sole purpose of this summary isto present some concepts of the invention in a simplified form as aprelude to the more detailed description that is presented hereinafter.

Described herein generally include surface modifications of polymericmembranes. More particularly, the doping of inorganic minerals, whichincludes embedment of inorganic particles and, in some instances, thegrowth of inorganic minerals on the embedded particles, to the surfaceof hydrophobic membranes is described herein. In other words, a methodhas been developed by which the inorganic materials can be stablyembedded into a polymer matrix and be functional in the desiredapplication. Furthermore, membrane hydrophilicity and other propertiescan be controlled through the selection and amount of doped materials.The need of surface modification of hydrophobic polymeric membranes bydoping inorganic materials is thereby successfully satisfied.

In one embodiment, the in-situ embedment of inorganic particles, whichare dispersed in a non-solvent bath, onto the membrane surface duringthe polymer precipitation and solidification. In another embodiment, anoptional process additionally involves the subsequent growth of newinorganic minerals onto the embedded particles.

To the accomplishment of the foregoing and related ends, the inventioncomprises the features hereinafter fully described and particularlypointed out in the claims. The following description and the annexeddrawings set forth in detail certain illustrative aspects andimplementations of the invention. These are indicative, however, of buta few of the various ways in which the principles of the invention maybe employed. Other objects, advantages and novel features of theinvention will become apparent from the following detailed descriptionof the invention when considered in conjunction with the drawings.

BRIEF SUMMARY OF THE DRAWINGS

FIG. 1 depicts a schematic illustration of the device for fabricatingmembranes by the wet phase separation technique, together with the“in-situ” surface embedment of inorganic particles. The inorganicparticles are dispersed in the non-solvent (e.g. water) phase during thepolymerization process.

FIGS. 2(a) and 2(b) are pictures of top surfaces of (a) theultrafiltration and (b) the microfiltration polyvinylidene fluoride(PVDF) membranes fabricated by the wet phase separation techniquewithout particle doping.

FIGS. 3(a) and 3(b) are pictures of top surfaces of agamma-alumina-embedded PVDF membrane showing (a) the particle-modifiedsurface and (b) the details of the anchored particles.

FIGS. 4(a) and 4(b) are pictures demonstrating the gibbsite growth onthe gamma-alumina-embedded surface. The result was achieved byhydrothermally (105° C.) treating the base membranes in a 3 mM aluminumsulfate solution for (a) 8 hours and (b) 2 days.

FIG. 5 illustrates a Table describing the enhanced membrane surfacehydrophilicity due to gamma-alumina particle embedment and thesubsequent gibbsite growth.

FIG. 6 depicts a schematic illustration of the hydrophilic surface'scontact angles.

DETAILED DESCRIPTION

The methods of surface modification of polymeric membranes involvedoping of inorganic minerals by in-situ embedment of inorganic particlesduring polymeric membrane solidification. Generally speaking, wet phaseseparation membrane formation process is employed to make the embeddedpolymeric membranes. A wet phase separation membrane formation processinvolves precipitating a dissolved polymer by immersion in a non-solventbath to form a membrane structure. That is, the process involves theimmersion of the cast solution of the polymers into a bath ofnon-solvent for the polymer precipitation and solidification.

The embedded polymeric membrane is made of any polymer that can be usedin a wet phase separation membrane formation process. Examples ofpolymers include one or more of polyvinylidene fluorides (PVDF),polysulfones (PS), polyethersulfones (PES), polyacrylonitriles (PAN),polyimides, and polyvinyl chlorides (PVC), polyphenylsulfones (PPES),cellulose nitrate, and cellulose acetate, and copolymers and terpolymersthereof.

A number of inorganic particles can be used for the embedment. Suitableinorganic particles impart or increase the hydrophilicity of the polymermembrane while occupying surface positions on the polymer membrane.Examples of inorganic particles include aluminum oxides, aluminumhydroxides, titanium dioxide, and silver particles. Specific examples ofalumina include gamma-alumina, eta-alumina, theta-alumina, and the like.

In one embodiment, the inorganic particles have an average particle sizefrom 5 nm to 500 nm. In another embodiment, the inorganic particles havean average particle size from 10 nm to 250 nm. In yet anotherembodiment, the inorganic particles have an average particle size from20 nm to 100 nm.

Requirements of this in-situ particle embedment approach arestraightforward. Firstly, the particles are well dispersed in thenon-solvent (e.g., water) bath, which can be easily attained bysufficiently reducing the particle size of dopants using conventionaltechniques. Secondly, the optimization of embedding particles to aspecific membrane can be predicted by calculating the change ofinterfacial energy for the attachment of particles on the targetmembrane.

As described herein, the term “cast solution” refers to a polymersolution in which the polymer is well dissolved in a suitable solvent.The cast solution can also contain a suitable amount of a non-solvent,additive, and/or co-polymer. The term “non-solvent” refers to as aliquid or mixed liquid in which the polymer coagulates and/orprecipitates.

Examples of solvents include one or more of N-methyl-2-pyrrolidone,dimethylformamide, dimethylacetamide, and the like. Examples ofnon-solvent, additive, and/or co-polymer include one or more ofaliphatic polyhydric alcohols such as diethylene glycol, polyethyleneglycol or glycerol; lower aliphatic alcohols such as methanol, ethanolor isopropyl alcohol; lower aliphatic ketones such as methyl ethylketone; water; and polyvinylpyrrolidone.

In one embodiment, the cast solution contains from 5% to 40% of polymer,from 10% to 90% of solvent, and from 1% to 40% of poor solvent,non-solvent, additive, and/or co-polymer. In another embodiment, thecast solution contains from 10% to 30% of polymer, from 20% to 80% ofsolvent, and from 2% to 30% of poor solvent, non-solvent, additive,and/or co-polymer.

Two specific examples of cast solutions were prepared as follows. Castsolution A comprises 18% PVDF (by weight), 3% glycerol (as poor solvent)and 79% N-Methyl-2-pyrrolidone (as the solvent for PVDF). Cast solutionB is identical to cast solution A except that it contains 10% (by weightof PVDF) of polyvinylpyrrolidone (as additive). The non-solvent for PVDFprecipitation was water, and the temperature of water can be increasedabove room temperature for increased membrane structure control.

The polymer membrane can be fabricated by the wet phase separationtechnique, by using the device as shown in FIG. 1. Both ultrafiltrationand microfiltration membranes can be fabricated by this technique. Thepolymeric membranes are for microfiltration, for ultrafiltration, or forthe supporting membranes of nanofiltration or of reverse/forwardosmosis. The polymeric membranes can have either isotropic oranisotropic structure.

The product examples are demonstrated in FIG. 2, with theultrafiltration membrane fabricated using cast solution A in a water(non-solvent) bath at room temperature and the microfiltration membranefabricated using cast solution B in a water (non-solvent) bath at 90° C.The in-situ particle embedment technique involved in the invention makesslight modification to the classical wet phase separation technique bydispersing the inorganic particle dopants in the non-solvent bath. Inour demonstration, nano-sized (average diameter of 40 nm) gamma-aluminaparticles were dispersed in the water bath at a concentration of 0.15g/L. The particles were embedded onto the PVDF membrane surface duringthe polymer precipitation and solidification (membrane formation)processes. The distribution of particles on the membrane is uniform(FIG. 3a ) and the particles are tightly anchored into the PVDF matrix(FIG. 3b ). This in-situ particle embedment technique is simple and easyto be adopted by current membrane fabrication facilities.

The invention extends to the growth of new inorganic phases on theembedded particles. Generally, the growth of new inorganic phases on theembedded particles is conducted under elevated temperatures in anaqueous solution of inorganic materials. In other words, additionalgrowth of other anhydrous or hydrated minerals such as aluminum oxidescan be accomplished.

In one embodiment, the growth of new inorganic phases is carried out inan aqueous solution at a temperature from 30° C. to 150° C. In anotherembodiment, the growth of new inorganic phases is carried out in anaqueous solution at a temperature from 40° C. to 100° C. In yet anotherembodiment, the growth of new inorganic phases is carried out in anaqueous solution at a temperature from 40° C. to 100° C. Examples of theinorganic materials are the same as or derivates (such as (hydr)oxidesof aluminum, oxides of titanium, and metal silver) of the inorganicparticles described above.

An example is shown by the growth of gibbsite (gamma-Al(OH)₃) on theembedded gamma-alumina particles by hydrothermally treating the membranein a 3 mM Al₂(SO₄)₃ solution at 105° C. (FIG. 4). The coverage ratio ofthe grown phase can be easily controlled by, but not limited to, theduration of the hydrothermal treatment. No inorganic substance was foundto stably grow on the bare membrane (without precedent particleembedment), which beneficially prevents the blockage of the membranepores and reveals the necessity of embedding the “root” particles on themembrane. The process of growing inorganic phases to further modify themembrane surface property is optional but provides the flexibility ofcreating the surface-root composite structure, based on the differentparticle materials anchored on the membrane surface.

The resultant embedded polymeric membranes contain embedded inorganicparticles, polymer, and optionally grown inorganic material (if thehydrothermal growth technique is exercised). In one embodiment, theresultant embedded polymeric membranes on the surface contains from 2%to 70% of inorganic particles, from 30% to 98% of polymer, and from 0%to 70% grown inorganic material. In another embodiment, the resultantembedded polymeric membranes contains on the surface from 5% to 60% ofinorganic particles, from 40% to 95% of polymer, and from 0% to 60%grown inorganic material.

Regardless of the precise manner in which inorganic particles areembedded in the surface of polymeric membranes, the term “surface” inthis context means to a depth of 0.5 microns or less. That is, the termsurface is from the superficial or outermost boundary of the polymericmembranes to a depth of 0.5 microns or less. In some instances, surfaceincludes to a depth of 0.25 microns or less. In other instances, surfaceincludes to a depth of 0.1 microns or less. In still other instances,surface includes only the exposed outermost boundary of the polymericmembranes (what is present on the superficial surface).

The increase of hydrophilicity of resultant embedded polymeric membranesis attributable to inorganic particles on the surface of the polymer. Inone embodiment, at least 25% area of the surface of the resultantembedded polymeric membranes is the inorganic particles. In anotherembodiment, at least 50% area of the surface of the resultant embeddedpolymeric membranes is the inorganic particles. In yet anotherembodiment, at least 60% area of the surface of the resultant embeddedpolymeric membranes is the inorganic particles.

The increase of hydrophilicity is apparent after the particle embedmentand the subsequent inorganic phase growth. In one embodiment, theresultant embedded polymeric membrane has contact angle of 80° or less.In another embodiment, hydrophilic surfaces have contact angles of 70°or less. In yet another embodiment, hydrophilic surfaces have contactangles of 60° or less.

Hydrophilicity refers to the physical property of a surface to like orattach water. Hydrophilicity can be described in more quantitative termsby using contact angle measurements. Referring to FIG. 6, the contactangle 9 is defined by the equilibrium forces that occur when a liquidsessile drop 3, 4 is placed on a smooth surface 2. The tangent 5, 6 tothe surface 2 of the convex liquid drop 3, 4 at the point of contactamong the three phases (solid, liquid, and vapor) is the contact angleθ₁, θ₂ as illustrated in FIG. 6. Young's equation, γ_(SL)=γ_(S)−γ_(L)cos θ defines the relationship between the surface tension of thesolid-vapor (γ_(S), vector along surface 2 away from center of drop 3,4), solid-liquid (γ_(SL), vector along surface 2 toward center of drop3, 4), and liquid-vapor (δ_(L), tangent 5, 6).

For purposes of this invention, hydrophilic surfaces have contact anglesof about 90° or less. In another embodiment, hydrophilic surfaces havecontact angles of increasing the hydrophilicity means decreasing thecontact angle, even if the decreased contact angle is more than 90°, forexample, a decreased contact angle from 120° to 95°.

Comparing the resultant embedded polymeric membranes with similarpolymeric membranes but not containing the embedded inorganic particles,the resultant embedded polymeric membranes have a contact angle at least10° less than the contact angle of the similar polymeric membranes butnot containing the embedded inorganic particles. In another embodiment,the resultant embedded polymeric membranes have a contact angle at least15° less than the contact angle of the similar polymeric membranes butnot containing the embedded inorganic particles.

An example is shown in the table of FIG. 5. The nascent membrane isquite hydrophobic with a water droplet contact angle of 94°. With theparticle embedment, the contact angle is reduced to 77°. Further contactangle decrease to 50° is observed with gibbsite growth.

The invention is not limited to only increasing the membranehydrophilicty. For example, nano silver particles are found to havebactericidal function and as such are beneficial for biofilm formationcontrol. Nano silver particles can be embedded onto the surface of anumber of membranes by dispersing the particles in the non-solvent bathfor the polymer of this invented technique. Another example is theemployment of this invention to embed titanium dioxide particles on asuitable membrane surface. As a type of photocatalysts, the embeddedtitanium dioxide will undergo photocatalytic reaction to achieve thepollutant degradation or detoxification under the irradiation of UV orsun light. Therefore, this invention is also providing a fabricationmethod for photocatalytically active membranes.

The embedded polymeric membranes have reduced fouling rates compared topolymeric membranes not containing the embedded inorganic particles. Forexample, using calcium alginate in a feed solution shows that the dopingof either gamma-alumina or gibbsite in PVDF membrane reduces the rate offormation of an undesirable gel layer on membrane surface. The additionof inorganic materials in embedding polymeric membranes leads toincreased membrane permeability and improved control of membrane-surfaceproperties. Therefore, a hydrophilic, less fouled and preferred propertyof polymeric membrane surface can be achieved to allow the use of watermembrane treatment technology for bioreactors and high solid-contentfeed water. The embedded polymeric membranes have usefulness in treatingwater having high organic solids, such as industrial waste streams,bioreactors, sewage, and landfill leachate.

Unless otherwise indicated in the specification and claims, all partsand percentages are by weight, all temperatures are in degreesCentigrade, and pressure is at or near atmospheric pressure.

With respect to any figure or numerical range for a givencharacteristic, a figure or a parameter from one range may be combinedwith another figure or a parameter from a different range for the samecharacteristic to generate a numerical range.

Other than in the operating examples, or where otherwise indicated, allnumbers, values and/or expressions referring to quantities ofingredients, reaction conditions, etc., used in the specification andclaims are to be understood as modified in all instances by the term“about.”

While the invention has been explained in relation to certainembodiments, it is to be understood that various modifications thereofwill become apparent to those skilled in the art upon reading thespecification. Therefore, it is to be understood that the inventiondisclosed herein is intended to cover such modifications as fall withinthe scope of the appended claims.

What is claimed is:
 1. A method of surface modification to polymericmembranes by doping of inorganic minerals; wherein the doping comprisesthe in-situ embedment of inorganic particles during membranesolidification in a non-solvent bath.
 2. The surface modification methodof claim 1, wherein the inorganic particles comprise one or moreselected from the group consisting of: aluminum oxides, aluminumhydroxides, titanium dioxide, and silver particles.
 3. The surfacemodification method of claim 1, wherein the polymeric membranes that arefabricated through a phase separation technique from one or moreselected from the group consisting of: polyvinylidene fluorides,polysulfones, polyethersulfones, polyacrylonitriles, polyimides, andpolyvinyl chlorides, polyphenylsulfones, cellulose nitrate, andcellulose acetate, and copolymers and terpolymers thereof.
 4. Thesurface modification method in claim 3 involves the immersion of thecast solution of the polymers into a bath of non-solvent for the polymerprecipitation and solidification.
 5. The surface modification method ofclaim 3, wherein the polymeric membranes are for microfiltration, forultrafiltration, or for the supporting membranes of nanofiltration or ofreverse/forward osmosis.
 6. The surface modification method of claim 3,wherein the polymeric membranes have either isotropic structure oranisotropic structure.
 7. The surface modification method of claim 1involves the dispersion of the inorganic particles in a non-solvent forthe polymeric materials of the membrane.
 8. The surface modificationmethod of claim 1, wherein the inorganic particles have an averageparticle size from 5 nm to 500 nm.
 9. The surface modification method ofclaim 1, wherein the cast solution comprises from 5% to 40% of polymer,from 10% to 90% of solvent, and from 1% to 40% of non-solvent, additive,and/or co-polymer.
 10. An embedded polymeric membrane, comprising on thesurface from 2% to 70% of inorganic particles, from 30% to 98% of apolymer, and from 0% to 70% grown inorganic material.
 11. The embeddedpolymeric membrane of claim 10, wherein the inorganic particles have anaverage particle size from 5 nm to 500 nm.
 12. The embedded polymericmembrane of claim 10, wherein the inorganic particles comprise one ormore selected from the group consisting of: aluminum oxides, aluminumhydroxides, titanium dioxide, and silver particles.
 13. The embeddedpolymeric membrane of claim 10, wherein the polymer comprises one ormore selected from the group consisting of: polyvinylidene fluorides,polysulfones, polyethersulfones, polyacrylonitriles, polyimides, andpolyvinyl chlorides, polyphenylsulfones, cellulose nitrate, andcellulose acetate, and copolymers and terpolymers thereof.
 14. Theembedded polymeric membrane of claim 10, wherein the embedded polymericmembrane contact angle of 80° or less.
 15. The embedded polymericmembrane of claim 10, where comparing the embedded polymeric membraneswith similar polymeric membranes but not containing the embeddedinorganic particles, the embedded polymeric membranes have a contactangle at least 10° less than a contact angle of the similar polymericmembranes but not containing the embedded inorganic particles.